I broadly concur with Stephen Borrill.

I use Xen 2, because that's all there was when I started and I haven't
moved to 3 yet.

I would run NetBSD 4.0_BETA2 (really, system built from netbsd-4) on
the dom0.  I'd be tempted to start with Xen 3.  The reason for these
choices is to avoid one upgrade.

NetBSD-current isn't stable enough.  Many moments it is just fine, but
doing a cvs update/rebuild/reinstall has the risk that you'll need to
expend effort to recover, and this doesn't seem reasonable for a
production machine.  Release branches generally do not have problems
with this update practices; I do it all the time on the netbsd-2 and
netbsd-3 branch and am now starting on netbsd-4.

On the domUs, you can can run any version that supports Xen; there's
no reason it has to match the dom0.  I use a different kernel file (in
dom0 root) for each domU, so I can upgrade them separately.  I have in
fact run different versions, e.g. recent netbsd-4 and current from a
year ago, and have had no problems.

For pkgsrc, I use the head, and use pkg_rolling-replace to update.
dom0 won't have much (but it needs python), so this doesn't hurt
much.  For the domUs, do whatever you'd do if it were a real machine;
the fact that it's Xen is irrelevant.  That said, the only two
reasonable choices for pkgsrc are the latest quarterly branch and the
head.

The head of pkgsrc (and thus all quarterly maintenance branches) is
intended to work with all currently-maintained versions of NetBSD, as
well as with other operating systems.  As long as you're talking
2/3/4/current, the head or the latest quarterly branch should be fine.


-- 
    Greg Troxel <gdt@ir.bbn.com>